In electronic devices that interface with audio devices through an a.c. coupling capacitance, a change in the DC state on either side of the a.c. coupling capacitor due to power-up, power-down or some other voltage transition can result in an audible artifact due to charging/discharging of the a.c. coupling capacitor, which causes current to flow through the a.c. coupling capacitor and the audio device. For example, codec systems are used to drive input and output devices, e.g. speakers, headphones, amplifiers, microphones, CD players. The I/O contacts of the codec generally connect through a.c. coupling capacitors to the particular input and output devices. When a codec is powered up or down, the change in voltage across the a.c. coupling capacitor causes a glitch which can be heard as an annoying “click”, “pop”, or “whump” sound if the device has an audio output, e.g. speaker, headphone. Even if the device is a CD player, amplifier or microphone, the sound may be heard if the subsequent components do not suppress it. The particular sound, “click”, “pop”, or “whump,” depends on the frequency content of charging current induced by the voltage transition across the a.c. coupling capacitor.
In one prior art attempt to overcome this problem, upon a power-up or power-down command, the common mode generator circuit of the codec was switched directly to the input/output contacts and the RC time constant of the common mode capacitance and impedance of the common mode generator modulated the “glitch,” but the “glitch” was still present and there was cross-talk introduced through the switches. Another shortcoming is that the network of these switches and the a.c. coupling capacitances and input/output devices compounded other RC time constants with the previously mentioned one, which introduced delays so that there was a moderated glitch followed, after a short delay, by a second moderated glitch when the drive amplifiers are turned on/off. Another approach simply used external circuits such as clamping circuits on the ground side of the a.c. coupling capacitors to suppress any electrical artifacts due to charging/discharging of those capacitors.